Sunday, April 20, 2008

The average density of surface seawater is 1.025 grams per millilitre at 4 degrees Celsius. When compared to fresh water, with a density of 1.0 grams per millilitre at 4C, seawater is denser and heavier. That's an average of course, since the density of seawater varies around the planet.

Using SI units the mass density of surface seawater ranges from about 1020 to about 1029 kilograms per cubic meter. That depends on several fundamentals but the two most important factors are the surface temperature and the percentage of salinity. Generally speaking, surface seawater has a saline level of anywhere from 3.1 to 3.8 percent, (although in around areas which discharge fresh water it can be considerably lower.) The average salinity of surface seawater is about 3.5 percent or 35 practical salinity units (PSU).

I know, I'm probably boring you with all this, but stay with me a little longer and this might just get interesting. (If you find alarm bells interesting.)

Here's another factor. Deep seawater, because of the increased pressure, can achieve a density of 1050 kilograms per cubic meter.

We all know that cold air falls and warm air rises. The same occurs with water. And we all know that something dense will fall faster than something less dense occupying the same space. Fresh water is more buoyant than salt water so it would make sense that it floats nearer the surface. However, it's water. It mixes with the salt-water and changes the salinity thus the density. Add too much fresh water and reduce the density of the sinking salt water and the velocity at which it sinks is significantly reduced.

Now the nitty-gritty. The surface seawater around Antarctica hovers between 33.8 to 34.5 PSU. Because it's cold and relatively dense, it sinks. As it sinks it becomes even more dense and the rate of sink is accelerated. That movement displaces the water below it and causes the deeper waters to move, creating a current. A big current.

The same thing happens off Greenland in the North Atlantic. Simply put, between the Antarctic and Greenland movements of deep, dense, bottom salt water, they constitute the major natural engines which cause the ocean conveyor belt which drives the ocean currents.

Without those ocean currents the distribution of global heat would be completely altered and the world would experience total chaos. Not just a little bit either. It would be a complete global disaster.

Steve Rintoul said his team found that salty, dense water that sinks near the edge of Antarctica to the bottom of the ocean about 5 km (3 miles) down was becoming fresher and more buoyant.

So-called Antarctic bottom water helps power the great ocean conveyor belt, a system of currents spanning the Southern, Pacific, Indian and Atlantic Oceans that shifts heat around the globe.

"The main reason we're paying attention to this is because it is one of the switches in the climate system and we need to know if we are about to flip that switch or not," said Rintoul of Australia's government-backed research arm the CSIRO.

"If that freshening trend continues for long enough, eventually the water near Antarctica would be too light, too buoyant to sink and that limb of the global-scale circulation would shut down," he said on Friday.

And if that global current engine stops it means all ocean currents will change. If the same thing happens off Greenland the flow of ocean currents which keeps the climate, as we know it, relatively stable, will come to a near or, possibly, complete halt.

Rintoul and his team aren't blaming global warming - yet.

Rintoul said his team are studying if faster melting of icesheets or sea ice is the source of the fresher water but he said it was too early to tell if global warming was to blame.

Over the coming months, his team will study oxygen isotopes collected from water samples.

"Oxygen isotopes act as a tracer of ice melt and that information should help pin down exactly what the cause of the freshening is in the deep ocean," said Rintoul, of the Antarctic Climate and Ecosystems Cooperative Research Centre.

It really doesn't matter whether you believe in global warming or not. This is happening. The cause at this point is almost irrelevant. The effect is going to change how you live.